Thanks Hugh and all who contributed to this thread. It has been most educational. Since a battery plate will dissolve in water it seems that there should be a min voltage under fast discharge that should be avoided.
The loss of SOC accuracy comes from charging and watching the voltage while charging. And if a person charges slowly the best accuracy is maintained. Darryl --- On Tue, 1/19/10, Hugh <[email protected]> wrote: > From: Hugh <[email protected]> > Subject: Re: [RE-wrenches] Capacity loss due to rapid discharge > To: "RE-wrenches" <[email protected]> > Date: Tuesday, January 19, 2010, 3:41 AM > > Re: [RE-wrenches] Capacity loss due to > rapid > discharge > Thanks, Kent, > > > It is clear then that high discharge rates have a big > effect on > the voltage, but only a small effect on the actual > battery > capacity. That was my instinct but it's good to > have it > confirmed. And this will be important information for > those who > use Peukert's Law 'naively' to conclude that a > battery has > dramatically lower capacity when discharged at higher > rates. (In > fact if it were true that capacity is reduced to this > extent it would > also have very serious implications for battery > efficiency.) > > > Battery voltage during discharge does depend heavily > on discharge > rate, and hence it is hard to assign a voltage that will > accurately > represent a 'good' end of discharge where the > battery will not be > damaged/worn out/ sulphated. > > > I have worked with Australian inverters that use > amphour > measurements to calculate the SOC as well as using voltage > set-points > as a back up. This seems laudable on the face of it, > but the > added complexity makes it very difficult to interpret why > the > generator is still running. (Is it low voltage, low > amphours, > high load, favourite time of day for generator, favourite > time of > month for a boost charge, failure to synch, manual start, > etc..... > ) > > > It's not easy to get an accurate calibration for > SOC based on > amphour logging, especially if the battery is wandering > between 50% > and 80% SOC over a period. This is the most efficient > zone of > operation, so it's not a bad state of affairs per > se. While it > is good to get up to full charge periodically, this will > involve a lot > of gassing and hence lost amphours/watthours in the > system. I > prefer not to do this with fossil fuels unless it seems > essential to > the health of the battery. > > > Anyway meantime I do have to assign genstart voltages > for a > couple of systems that function automatically, and I will > probably > continue to use values around 23.5 volts (11.7, 47 volts) > for this > purpose. There's a bit of guesswork involved > because of the > unknown current and temperature, but that's a value > that has worked > well enough in the past. I am running a poll on my > blog to see > what other people choose. > http://scoraigwind.blogspot.com/ It's > not a very well defined question, but so far the answers do > peak at 47 > volts so I am not alone in my choice. > > > Using a voltage rather than SOC for such purposes > (user guidance, > genstart, etc) does have the advantage that it combines SOC > and energy > use into one parameter. If the user is hitting the > battery too > hard for the present conditions then the voltage will tend > to hit the > chosen 'danger level' and this can be a warning to > back off or to > start the backup. > > > Thanks again for all the input to this thread. > > > best > > > Hugh > > > At 11:47 -0800 18/1/10, Kent Osterberg wrote: > Hugh, > > > You may be interested in this > article > "A critical review of using the Peukert equation for > determining > the remaining capacity of lead-acid and lithium-ion > batteries" by > Dennis Doerffel and Suleiman Abu Sharkh from the School of > Engineering > Sciences, University of Southampton. It is available > from http://www.sciencedirect.com/science for a > small fee. > If the link does not work, just search for the lead > author's last > name. > > > > The authors describe testing lead acid batteries at high > rates of > discharge from fully discharged down to the point that the > terminal > voltage is 10.0 volts. After letting the batteries > rest, they > continued to discharge further at a lower rate until the > terminal > voltage was again 10.0 volts. Results were compared > to > discharging at the slow rate only. The total amphours > delivered > when a low discharge rate follows a high discharge were > less by 5 to > 10%. With 10% associated with a C2 and C/20 discharge > of a 17 AH > battery and 5% associated with a C/1.2 and C/13 discharge > of a 65 ah > battery. > > > In short, the capacity loss indicated by Peukert only > applies to a > continuous discharge rate. When a slow discharge > follows a rapid > discharge, the total number of amphours delivered is almost > the same > (just 5 to 10% less) as if the discharge happened at > entirely at the > slow rate. If you were estimating how far your > electric car > would travel, that 5 or 10% may be critical. For the > rates of > discharge and depths of discharge normally used for > off-grid homes the > "lost" capacity is probably even less. > > > > Kent Osterberg > > Blue Mountain Solar, Inc. > > > > > _______________________________________________ > > List sponsored by Home Power magazine > > > > List Address: [email protected] > > > > Options & settings: > > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > > > List rules & etiquette: > > www.re-wrenches.org/etiquette.htm > > > > Check out participant bios: > > www.members.re-wrenches.org > > > > > -- > > Hugh Piggott > > > > Scoraig Wind Electric > > Scotland > > http://www.scoraigwind.co.uk > > > -----Inline Attachment Follows----- > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: [email protected] > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > > _______________________________________________ List sponsored by Home Power magazine List Address: [email protected] Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
